The present invention concerns the baskets or supports used in the handling of silicon wafers in the semiconductor industry
The wafers, which are very thin disks which can have a diameter of between 12 and 20 cm, are treated by immersion in a number of successive baths which are very corrosive and some of which are also very hot, for example of the order of 200.degree. C.
In order to carry out this processing, the wafers are held in baskets, similar to drainage racks, which are transferred from one bath to another. These baskets are made from an inert injected plastic which is resistant to high temperatures, to the corrosive effect of the baths and which does not introduce impurities into the baths. Usually a plastic such as "Teflon-PFA" (a polytetrafluoroethylene resin) is used.
Very expensive components are formed from the silicon wafers, and this is why every precaution is taken to ensure maximum purity during the treatment.
In effect the presence of impurities in the baths can compromise the quality of the components fabricated, leading to a high waste load which can prove very costly. It is important therefore that the baskets themselves do not contribute to the pollution of succesive baths. For this reason the baskets are moulded in a controlled atmosphere in perfectly polished moulds, and their form is such as to allow the complete rinsing and drainage of the basket and the wafers supported therein.
Indeed, if traces of a previous bath are retained in surface flaws and thereby transferred to a subsequent bath, the latter can become contaminated and spoil the whole batch.
In spite of measures taken to avoid surface faults on the baskets, there remains the faults inherent to the injection process. These faults tend to be situated in zones where there are internal tensions, notably at the injection sills.
These tensions are due to:
turbulent flow of the material at the level of the injection sill,
a temperature difference between the material inside the piece and the material in contact with the mould,
an over injection necessary to compensate for the contraction that takes place during cooling,
the fact that the feed troughs hinder the shrinkage of the piece or that shearing of the material takes place at the junction between the feed troughs and the piece.
These internal tensions are released in course of time through the effect of thermal shocks or through the corrosive action of the baths. This leads to the formation of cracks which increase in size with the basket's use and which retain particles from successive baths. After moulding of the basket, the injection cluster is cut, but the internal tensions situated around this point remain in the piece. Added to this, the cut cannot be made exactly in the plane of the piece, there being always a small part jutting out or else a small hollow. In both cases there is a fault which harbours particles, and consequently, acts as a source of contamination for the baths.